1. Technical Field of the Invention
This invention relates generally to communication systems and more particularly to Digital Subscriber Line (DSL) based communication systems.
2. Description of Related Art
Communication systems are known to enable a plurality of communication devices to communicate among themselves and with communication devices in other communication systems. Such communication devices, which may be computers, modems, facsimile machines, printers, personal digital assistants, et cetera, communicate voice, text, and/or video data. Such communication systems support the communication of data in accordance with one or more communication standards. As is known, there are a large number of communication standards for the communication of data and vary from country to country, including a plurality of standards governing digital subscriber line (DSL) communications. For example, the United States, Europe, Japan, China and other countries each have standards for various types of DSL based communications, including, but not limited to, asynchronous digital subscriber lines (ADSL) and very high bit rate digital subscriber line (VDSL).
As is also known, for a communication device at a customer premises to participate in a DSL based communication, the communication device includes a DSL modem and communicates with a DSL modem at a central office. The DSL modem at the customer premises is coupled to the DSL modem at the central office via a DSL link (or loop) that typically is comprised of an unshielded pair of wires within a multiple pair cable (i.e., a bundle of pairs of wires).
Due to the usage of multiple pair cables, the length of the DSL loop is limited by mutual interferences between the pairs of wires within the same cable. This interference is generally known as cross-talk, which causes errors in the received signal and thus reduces performance of the DSL modem. Such cross-talk is either near end cross-talk (NEXT) or far end cross-talk (FEXT).
As is known, far end cross-talk of a loop in a multiple pair cable is proportional to the length of a loop and the loop transfer function. As the loop length increases (e.g., greater than 200 meters), the attenuation of the loop, which is an inverted logarithm of the loop transfer function, increases exponentially with respect to the length. At and above such lengths, the attenuation of the loop becomes the dominant factor and attenuates the far end cross-talk.
In a typical access network, DSL links originate at the Central Office (CO) and terminate at the Customer Premises (CPE), which are located at different distances from the Central Office. As such, CPEs coupled to the CO via shorter length loops generate significant far end cross-talk is significant, which, reduces signal-to-noise ratio (SNR) at DSL modems of the central office serving longer length DSL links. The reduced SNR correspondingly reduces upstream (i.e., from the customer premises to the central office) performance of these modems, i.e., forces a lower bit rate, increases error rate, etc.
One known method to reduce far end cross-talk generated by shorter loops is to reduce the transmit power, or power spectral density, for upstream transmissions based on the length and attenuation of the particular loop. To do this, the length of the loop must be determined, which can be done by transmitting a single known signal from the DSL modem at the central office to the DSL modem at the customer premises. The customer premises modem determines the attenuation of the loop based on the power level of the received signal and the known power level the transmitted signal. The customer premises modem then determines the loop length by estimating its electrical loop length obtained by dividing the attenuation of the loop by a reference attenuation value. The reference attenuation value may be obtained by taking the square root of the frequency of the known signal or by some other function relating to the frequency (f) of the known signal; for example: the reference attenuation value may be equal to α+√f, or α+√f+β×f, where α and β are coefficients. Alternatively, multiple signals could be transmitted yielding multiple estimated lengths, which are then averaged to achieve the final estimated electrical length.
In either of these methods for estimating the electrical loop length, errors result in an over estimation of the loop length. Such errors occur because the loop length estimation method does not take into account inaccurate terminations at the end of the wires, mixed wire gauges, bridge taps (unloaded wire drops from the loop for another customer premises) water penetration, improper splicing, et cetera. When the loop length is over estimated, the transmit power is not sufficiently reduced, thus the far end cross-talk is too large and continues to adversely affect the performance of the other DSL modems of the CO in the multi-pair cable.
Therefore, a need exists for a method and apparatus that accurately estimates the electrical loop length and for applications thereof to reduce transmit power of DSL modems.